PHILIP C. JESSUP DIES; HELPED END BERLIN BLOCKADE

Philip C. Jessup, an American authority on international law who was credited with a key role in ending the Soviet blockade of West Berlin, died yesterday at his winter home in Newtown, Pa. He was 89 years old.

Mr. Jessup, who was for many years a professor at Columbia University, had been suffering from Parkinson's disease. He had maintained a residence in Norfolk, Conn., since the early 1930's and kept an apartment in Manhattan during much of his career.

Mr. Jessup, a rangy 6-footer with bushy eyebrows, a wide brow and a scholarly mien, was a United States representative to the United Nations General Assembly from 1948 to 1952 and a member of the International Court of Justice in The Hague from 1960 to 1969.

He also served as a United States Ambassador at Large from 1949 until his resignation in 1953 and was one of the closest advisers of Secretary of State Dean G. Acheson at the time.

While he was serving at the United Nations, he played a crucial role in intricate diplomatic maneuverings leading to the end of the Berlin blockade. He was instructed by Washington to approach a Soviet delegate and ask whether a new twist in a Stalin declaration - the omission of a usual reference to the use of Western currency by the United States, Britain and France in Berlin - indicated a Soviet willingness to begin negotiations. The currency issue was a bone of contention that had been much emphasized by the Russians earlier.

In a chat at a bar at the United Nations, the Soviet diplomat, Yakov A. Malik, said he did not know but would inquire. A week later, he told Mr. Jessup that Stalin's statement was indeed important, and after further diplomatic inquiries, the two powers embarked on the negotiations that eventually led to the blockade's end.

Mr. Jessup was praised in a New York Times editorial for his ''brilliant work in ending the Russian blockade.'' Attacked by McCarthy

At the time, Mr. Jessup was attacked by Senator Joseph R. McCarthy for having ''an unusual affinity for Communist causes.''

The Times called the attacks vicious and quoted what Gen. Dwight D. Eisenhower, as president of Columbia University, had written in a letter to Mr. Jessup: ''No one who has known you can for a moment question the depth or sincerity of your devotion to the principles of Americanism.''

After returning to teaching at Columbia for several years, Mr. Jessup was elected to the International Court of Justice by the General Assembly in November 1960. He served for nine years on the Court, the main judicial organ of the United Nations, which is better known as the World Court.

In 1966, when black African nations' legal attack on apartheid in the World Court failed, he wrote a long and stinging dissent from the decision.

In its ruling, the Court dismissed a complaint by Ethiopia and Liberia against the imposition of apartheid in the territory of South-West Africa, also known as Namibia. The decision was made on the technical ground that Ethiopia and Liberia did not have sufficient legal interest in their claim. 'Procedure of Utter Futility'

Justice Jessup, in his dissent, called the case ''a procedure of utter futility.''

Once during his nine years at the Court, he told an interviewer: ''When we have a case, the work is as hard as any I've ever done. Harder, perhaps, becuse we have to do all of the work ourselves. Unlike a national court, we have to work our way through an immense amount of documentation.''

Musing about the United Nations, he said: ''People expect too much of the U.N., and that's why they're disappointed. It is obvious that the U.N. by itself cannot arrive at any final solution to the Middle East problem.''

Philip Caryl Jessup was born Jan. 5, 1897, in Manhattan, one of the five sons of Henry Wynans Jessup, a professor of law at New York University, and the former Mary Hay Stotesbury. He spent much of his boyhood in Manhattan, graduated from the Ridgefield School in Connecticut, and went on to Hamilton College, where he graduated in 1919 with a bachelor's degree, after service with the United States Army in France and Belgium during World War I. He first went into banking, as assistant to the president and assistant cashier of the First National Bank in Utica, N.Y., from 1919 to 1921. He then earned a law degree from Yale Law School in 1924, a master's degree from Columbia, also in 1924, and a doctorate from Columbia in 1927, for which he wrote a thesis about the law of territorial waters and maritime jurisdictions.

He was admitted to the District of Columbia bar in 1925, during a two-year tour of duty as assistant solicitor in the State Department, and to the New York bar in 1927. He was a member of the law firm of Parker & Duryea from 1927 to 1943. Career at Columbia

He was also a lecturer on international law at Columbia University from 1925 to 1927, an assistant professor from 1927 to 1929, an associate professor from 1929 to 1935, a full professor from 1935 to 1946, and Hamilton Fish Professor of International Law and Diplomacy from 1946 to 1961. He was Jacob Blaustein Lecturer at Columbia in 1970.

In addition, he was a legal adviser to various Federal officials from 1924 to 1953.

His books included ''Elihu Root,'' a two-volume biography that came out in 1938, and ''A Modern Law of Nations,'' published in 1948, which Associate Justice Robert H. Jackson of the United States Supreme Court, writing in The New York Times, said might well ''become outstanding among such influences in reshaping the law of nations more nearly to accord with our aspiration for international justice under law.''

Other books by Mr. Jessup were ''Transnational Law'' (1956), ''The Use of International Law'' (1959), ''The Price of International Justice'' (1971) and ''The Birth of Nations'' (1974).

Over the years, he was variously a trustee of Hamilton College, the Council on Foreign Relations, the Foreign Policy Association and the Carnegie Endowment for International Peace, where he was also for a time director of the international law division.

Mr. Jessup is survived by his wife, the former Lois Walcott Kellogg of Utica, whom he married in 1921; a brother, Richard S. Jessup, of Hightstown, N.J.; a son, Philip C. Jessup Jr., of Washington, three grandchildren, and four stepgrandchildren.

No funeral is planned. The family requests that in lieu of flowers, contributions be made to the American Friends Service Committee in Philadelphia.

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A version of this obituary appears in print on February 1, 1986, on Page 1001013 of the National edition with the headline: PHILIP C. JESSUP DIES; HELPED END BERLIN BLOCKADE. Order Reprints|Today's Paper|Subscribe